Apparatus and circuitry for stabilizing laser diode output for analog modulation

ABSTRACT

Circuitry and apparatus for stabilizing the output of a laser diode when modulated with a wideband analog signal. The circuitry provides for continuous operation of the laser diode with a pin diode positioned for receiving a portion of the output from the laser diode via optics positioned between the laser diode and the pin diode. The optics includes a collimator, a linear polarizer and a polarization beam splitter. The circuitry includes a high gain amplifier which receives the analog signal with a compensating network connected between the amplifier output and the laser diode. The circuitry also has a feedback circuit portion including the pin diode connected to one input of the amplifier.

BACKGROUND OF THE INVENTION

The invention presented herein relates to arrangements for stabilizationof the output of an injection laser (laser diode) when modulated with awideband analog signal.

An acceptable laser output stabilization arrangement must correct forlong and short term changes in laser diode operating characteristics ifthe laser diode is to be usable for modulation by a wideband analogsignal. Over the long term, the slope of the curve of laser diode poweroutput versus input current tends to decrease with the age of the laserdiode. In addition, the lasing threshold current tends to increase withage.

Short term variations in the laser diode operating characteristics canoccur in response to junction temperature changes. Junction temperaturechanges due to self-heating or ambient temperature fluctuations cancause continuous changes in the power output of the laser diode forfixed input current. These continuous changes are equivalent to a shiftto the right of the power-current curve with increasing junctiontemperature.

In addition to the continuous changes, some single-mode laser diodesexhibit abrupt power-output fluctuations of a few percent of maximumpower due to "hopping" between longitudinal modes of the laser cavity.In a given external optical setup, these mode hops occur at particularjunction temperatures, with the junction temperature being dependentprimarily on the recent history of junction self-heating.

Various feedback arrangements for stabilizing the laser diode output aredisclosed in the prior art which involve monitoring the output of thelaser diode by directing a portion of the light emitted from the frontor back facets of the laser diode to a pin diode. Some laser diodes areprovided with an internal diode for such use. In the case of laserdiodes driven from digital data sources, the feedback arrangements areprovided to maintain average power levels. Such arrangements, of course,only provide correction for long term changes in laser diode operatingcharacteristics and do not provide immediate or continuous correctionfor abrupt power-output fluctuations due to "hopping" betweenlongitudinal modes of the laser cavity or power level changes due toshort term temperature fluctuations.

A paper entitled "Modulated Light Source For Recording WithGaAlAs-Lasers" by M. Lutz, B. Reimer, and H. P. Vollmer which waspresented at the "First International Congress on Advances in Non-ImpactPrinting Technologies" held at Venice, Italy in June 1981, discloses anelectronic circuit employing the pin diode that is built into the laserdiode assembly. A feedback arrangement is described wherein the pindiode monitors the light output from the back mirror of a laser diode tostabilize the light output of the laser diode. This paper recognizes theneed for rapid stabilization of the laser output for printerapplications. A control circuit is disclosed which keeps the laser diodeoperating all the time so a feedback signal is always present and isindicated as providing a feedback loop having a time constant that ismuch less than 300 nanoseconds. The rise and fall times for this circuitare given as 150 and 300 nanoseconds, respectively.

While the prior art appears to teach how one might obtain regulation ofthe output of a laser diode making it suitable for non-impact printers,such teachings fall short with respect to how satisfactory control ofthe laser diode output can be obtained for applications using analoginput signals wherein feedback corrections in less than 40 nanosecondsare desired. The prior art also fails to teach a solution to the problempresented due to mode "hopping" between longitudinal modes of the lasercavity.

SUMMARY OF THE INVENTION

The invention presented herein represents an advance over the prior artwith respect to several aspects or features that are employed in oneembodiment of the invention which provides for analog modulation of alaser diode at rates up to several million samples per second withprecise control over the power output that is provided in part by afeedback arrangement wherein corrections are made in less than 40nanoseconds with a closed-loop rise time of less than 50 nanoseconds.

The attainment of such control of the laser diode output is provided inpart by the present invention by the manner in which a portion of thelaser diode light power output is obtained for use in developing afeedback signal. Since continuous control of the output is sought, theproblem presented by abrupt power output fluctuations due to "hopping"between longitudinal modes of the laser cavity must be addressed. It hasbeen discovered that this problem is minimized by the present inventionwherein a linear polarizer is positioned before a polarization beamsplitting element to provide a portion of the laser diode main output toa pin diode with the axis of the linear polarizer aligned parallel tothe axis of polarization of the light from the laser diode. This allowsthe use of a very efficient polarization beam splitter to split off thefeedback portion of the output beam at 90 degrees to the main outputbeam. With this arrangement polarization angle changes due to mode"hopping" are presented as intensity changes as far as optics downstreamfrom the linear polarizer are concerned for which correction can be madeby the use of the circuitry of the present invention.

Another feature of the invention resides in the feedback circuitry thatuses detection of the photo-induced current produced by the pin diode.The circuitry provided by the invention serves essentially to cancel theadverse effect of the junction capacitance of the pin diode on the speedat which such circuitry responds to changes in the pin diode current.

The circuit provided for control of the laser diode output also uses ahigh speed gain element, a constant-current source for supplying thelaser diode with a quiescent current at a point corresponding to abouthalf of the maximum power output, and a compensating network forincreasing circuit stability and speed.

BRIEF DESCRIPTION OF THE DRAWING

The novel features and advantages of the invention presented herein willbecome more apparent to those skilled in the art upon consideration ofthe following detailed description and referenced drawing wherein:

FIG. 1 is a schematic of one embodiment of the invention wherein theoptics apparatus portion is shown as a single block;

FIG. 2 is a block diagram showing more detail of the optics used in theembodiment of FIG. 1; and

FIG. 3 is a closed-loop gain versus frequency response for theembodiment of FIG. 1.

DETAILED DESCRIPTION

Referring to FIG. 1 of the drawing, details of the circuitry portion forstabilization of the output of a laser diode 10 are shown. FIG. 2 showsdetails of the apparatus portion, indicated by the block 12, whichinvolves the optics for directing a portion of the main beam output ofthe laser diode 10 to the pin diode 14 used in the circuitry of FIG. 1.

The circuitry of FIG. 1 includes a voltage input bias network 16 that isprovided by two series connected resistors 18 and 20 with resistor 18connected to a positive d.c. voltage and resistor 20 connected toground. The connection common to the resistors 18 and 20 receives ananalog input signal for control of the laser diode 10. The bias network16 provides a small d.c. offset to the circuit so the laser diode outputpower will not be turned off, but will drop to its threshold level whenthe analog input signal goes to zero. A low-pass filter 22 is providedby a resistor 24 and capacitor 26 in conjunction with the impedancelooking back at the input to the circuit of FIG. 1. One end of resistor24 is connected to the connection common to resistors 18 and 20 with theother end of resistor 24 connected to one side of capacitor 26, whichhas its other side connected to ground. The low-pass filter serves toimprove the closed-loop response of the circuitry in FIG. 1 in that itreduces residual peaking to expand the high end of the frequency bandover which a flat gain is provided. A high-gain differential amplifier28 is provided which has its inverting input connected to the connectioncommon to capacitor 26 and resistor 24 with its non-inverting inputconnected to receive a feedback signal for stabilizing the output of thelaser diode 10. The output of the differential amplifier 28 is connectedto a capacitor 32 that is connected in parallel with a resistor 30 whichprovides a compensating network for increasing the speed and stabilityof the circuitry. A current source, indicated at 34, is connected to thecathode of the laser diode 10 and is also connected to the output end ofthe parallel connected resistor 30 and capacitor 32. The current source34 includes an NPN transistor that is biased to provide the laser diode10 with a current, when the output of amplifier 28 is zero, that isequal to the level required for half of the maximum output power of thelaser diode. This means the current output that otherwise would berequired by the amplifier 28 is reduced to provide a larger selection ofhigh gain amplifiers that are usable in the circuit of FIG. 1. Thecurrent source 34 need not be used if an amplifier 28 is selected thathas the required output current rating.

Current produced by the pin diode 14 in response to the light outputreceived from the laser diode 10 via the optics portion 12 istransformed to a positive feedback voltage by the circuit portion 36.The circuit portion 36 minimizes the effects of the pin junctioncapacitance thereby extending the frequency response of the pin diode.The portion 36 includes a high speed unity gain buffer amplifier 38, aresistor 40, a resistor 42 and a d.c. blocking capacitor 44. Amplifier38 has an input connected to the anode of the pin diode 14. A currentpath through the pin diode 14 is provided by resistors 40 and 42.Resistor 40 is used to sense the current in the pin diode and isconnected between ground and the anode of the pin diode 14. Resistor 42is connected between a positive d.c. voltage and the cathode of the pindiode. The d.c. blocking capacitor 44 is connected between the pin diode14 end of the resistor 42 and the output of amplifier 38 which connectsto the non-inverting input of the differential amplifier 28. Theresistor 42 and capacitor 44 serve substantially to reduce the voltagevariations across the pin diode 14. The RC time constant provided byresistor 42 and capacitor 44 should be made much greater than any othertime constants in the circuit portion 36. With this condition and for again of one for the amplifier 38, the response speed of the circuitportion 36 is optimized.

The compensating network provided by the resistor 30 and capacitor 32parallel combination uses a value for resistor 30 that is much greaterthan the sum of the output impedance of the amplifier 28 and the dynamicimpedance of the laser diode 10. The open-loop gain of the circuitry ofFIG. 1 is then approximately inversely proportional to the magnitude ofresistor 30. The resistor 30 also serves as a current limiter for thelaser diode 10. The value for capacitor 32 is selected so as to providea zero in the open-loop gain which cancels out a pole due to theresponse of the pin diode 14.

Referring to FIG. 3, the closed-loop gain versus frequency for thecircuitry embodying the invention presented herein is shown wherein thegain is essentially flat with the 3b point on the curve being at about20×10⁶ Hertz.

Referring to FIG. 2, the optical apparatus used between the laser diode10 and the pin diode 14 includes a collimator 46, a linear polarizer 48and a polarization beam splitter 50. A lens required to focus the lighton the pin diode 14 is not shown. The output of the laser diode 10 isdirected to the collimator 46 with the output of the collimatorpresented to the linear polarizer 48 which has its output directed tothe polarization beam splitter 50. The beam splitter 50 is used todirect a portion of the output from the polarizer 48 to the pin diode 14at an angle of ninety degrees to the main beam. The amount of lightsplit off for the pin diode 14 can be adjusted by rotating thepolarization beam splitter 50. The polarization axis of the linearpolarizer 48 is aligned parallel to the axis of polarization of thelight from the laser diode 10. The arrangement of FIG. 2 avoids theproblems associated with arrangements using light from the back facet ofthe laser diode for the pin diode when continuous monitoring of thelaser diode output is desired. One problem that is avoided is the changethat occurs in the proportionality constant relating to the light poweroutputs between front and back facets of a laser diode. The problempresented by differences in intensity fluctuations between the front andback facets when there are intensity fluctuations due to mode "hopping"is also avoided by the FIG. 2 arrangement wherein a portion of the lightfrom the front facet of the laser diode is directed to the pin diode.The present arrangement, which uses the linear polarizer 48 upstream ofthe polarization beam splitter 50, also avoids increases and decreasesin the feedback portion of the laser diode light output at the expenseof the remaining portion of the beam due to polarization angle changeswhich can occur along with intensity fluctuations caused by mode"hopping". If the linear polarizer 48 provided by the arrangement ofFIG. 2 were not used, any alteration of the polarization angle due tomode "hopping" would cause the proportion of light that is directed tothe pin diode 14 to be altered. If the polarization angle change is inthe direction that reduces (increases) the proportion of the lightreceived by the pin diode, the usable portion of the light is increased(decreased). The feedback circuitry would then respond to a reduction(increase) in the light received by the pin diode to increase (decrease)the output of the laser diode when the output of the laser diode shouldactually be decreased (increased). By using the linear polarizer 48 anypolarization angle change due to mode "hopping" merely causes a changein the light intensity from the linear polarizer 48 and does not alterthe proportion of the light that is received by the pin diode 14 so thatany polarization angle changes due to mode "hopping" are seen only asintensity changes as far as the optics downstream from the linearpolarizer are concerned. Proper correction for intensity changes will bemade in the laser diode output by the feedback circuitry.

While the invention has been described in connection with an exemplaryembodiment thereof, it will be understood that many modificationsthereof will be readily apparent to those of ordinary skill in the art;and that this specification is intended to cover any adaptations orvariations thereof. Therefore, it is manifestly intended that thisinvention be only limited by the claims and the equivalents thereof.

What is claimed is:
 1. Circuitry and apparatus for stabilizing theoutput of a laser diode allowing the laser diode to be modulated by awideband analog signal including:a laser diode; means operativelyarranged for receiving the analog signal for modulating the output ofsaid laser diode, said means including a high gain amplifier having twoinputs, one input operatively connected for receiving the signal formodulating said laser diode, the other input for receiving a feedbacksignal; a compensating network connecting the output of said high gainamplifier to said laser diode; a feedback circuit connected to provide afeedback voltage signal to said other input of said high gain amplifier,said feedback circuit including a pin diode; means for opticallycoupling said pin diode to the laser diode for providing a portion ofthe light output from said laser diode to said pin diode.
 2. Circuitryand apparatus according to claim 1 wherein said feedback circuitincludes a resistor, capacitor and a unity gain buffer amplifierconnected in a positive feedback arrangement to minimize the effects ofthe junction capacitance of said pin diode, thereby extending thefrequency response.
 3. Circuitry and apparatus according to claim 1wherein said compensating network includes a resistor connected betweenthe output of said high gain amplifier and said laser diode and acapacitor connected in parallel with said resistor, said resistor havinga value that is greater than the sum of the output impedance of saidhigh gain amplifier and the dynamic impedance of said laser diode andsaid capacitor having a value that provides a zero in the open-loop gainfor cancelling a pole due to the response of said pin diode. 4.Circuitry and apparatus according to claim 1 wherein saidfirst-mentioned means includes a bias network operatively connected tosaid one input of said high gain amplifier for providing a d.c. offsetsufficient to keep said laser diode operating with said laser diodeoutput dropping to its threshold level when the analog signal formodulating said laser diode goes to zero.
 5. Circuitry and apparatusaccording to claim 1 wherein said last-mentioned means includes acollimator, linear polarizer and a polarization beam splitter positionedin that order between said laser diode and said pin diode for directinga portion of the light output from said laser diode to said pin diodewith said collimator positioned to receive the light output from saidlaser diode.
 6. Circuitry and apparatus according to claim 5 wherein theaxis of said linear polarizer is aligned parallel to the axis ofpolarization of light from said collimator.